1. Field of the Invention
This invention relates to a gas sensor. More particularly it relates to a gas sensor capable of sensing and detecting various combustible gases such as carbon monoxide, propane gas, town gas and hydrogen.
2. Description of the Prior Art
When the various combustible gases exist in an atmosphere, there are toxicity to organisms and danger of explosion. Therefore, various gas sensors capable of detecting the gases have been proposed.
The known gas sensors exploit resistance variations, oxidizing reaction heat, electrochemical reactions, etc.
A typical gas sensor utilizing electrochemical reactions among the prior-art gas sensors is disclosed in U.S. Pat. No. 4,005,001 and Japanese Patent Application Publication No. 14639/1977.
This gas sensor is such that electrodes made of materials whose catalytic activities to oxygen and combustible gases are different from each other are disposed on the surfaces of a solid electrolyte which has an oxygen ion-conductivity. When the gas sensor having such a structure is placed in a gas containing both the combustion gas and oxygen, an electromotive force corresponding to the concentration of the combustible gas is generated across both the electrodes because the electrodes have the different catalytic activities as described above.
This sensor, however, has the disadvantage that since the materials of the electrodes deposited on both the surfaces of the solid electrolyte are different, the electrodes exhibit different secular changes, so the characteristics of the sensor undergo conspicuous secular changes.
The detection sensitivity is approximately 1% in case of, for example, CO, and it is difficult and very unsatisfactory in practical use to detect a smaller quantity of CO. Moreover, the operating temperature is as high as about 650.degree. C., which leads to problems on the lifetime, the power dissipation etc.